Mammalian target of rapamycin complex 2 (mTORC2) kinase controls both cell proliferation and survival, but how mTORC2 is regulated is not well understood. This paper shows that phosphorylation of SIN1, an mTORC2 component, results in mTORC2 inactivity. SIN1 phosphorylation by either S6 kinase or AKT leads to its dissociation from mTORC2 and inhibition of growth factor-mediated activation of mTORC2. Cancer-associated mutations in SIN1 that impair its phosphorylation result in hyperactivity of mTORC2 and activation of AKT.